In electrodeposition coating using a cationic electrodeposition coating composition, attempts have been made to maintain the inner film thickness of bag portions and gap portions to ensure anti-corrosion properties, and to optimize or make uniform the outer film thickness (for example, to ensure the film thickness of a portion where finish and anti-corrosion properties are valued) for improving the quality of automobile bodies and reducing the cost.
However, in a coating film obtained by electrodeposition using a conventional cationic electrodeposition coating composition, a reduction in the coating film thickness may result in a poor finish due to an uneven substrate and a reduction in heat flow properties. Further, for the purpose of improving the finish, if the amount of plastic components used in the coating film is increased to improve heat flow properties during heating and drying, polarization resistance is reduced, whereby the “throwing power” and anti-corrosion properties of the coating film tend to be lowered.
In view of these circumstances, the development of a cationic electrodeposition coating composition that has excellent throwing power and that can form a thin coating film (in particular, having a dry film thickness of 10 μm) with excellent finish (heat flow properties) and anti-corrosion properties has been desired.
Patent Document 1 discloses that a cationic electrodeposition coating composition comprising an amino group-containing modified epoxy resin (A), a phenolic hydroxyl group-containing resin (B), and a blocked polyisocyanate curing agent (C) has excellent throwing power, and can form a coating film with excellent finish, in particular, a coating film having a dry film thickness of 15 μm, with excellent finish and anti-corrosion properties. However, the disclosed composition is not fully satisfactory in terms of all of throwing power, finish (heat flow properties) and anti-corrosion properties of a thinner coating film (in particular, having a dry film thickness of 10 μm).